This invention relates to optical wavelength add/drop multiplexers. More specifically, it relates to an optical wavelength add/drop multiplexer operable to add or drop digital optical signals from optical channels that may each be operating at one of two or more data transmission rates.
Broadband telecommunications networks are being configured to carry increasing volumes of voice, data and multimedia information. To meet these increasing volume demands, such networks are being implemented using optical communications systems technology. For example, optical wavelength-division multiplexed (WDM) technology may be used to support dozens of communications channels transported at different wavelengths on a single optical fiber.
In WDM optical networks, wavelength add/drop multiplexers (WADMs) have been used to selectively remove and reinsert WDM channels at intermediate points across these networks (see, e.g., C. Randy Giles et al., xe2x80x9cThe Wavelength Add/Drop Multiplexer for Lightwave Communications Networks,xe2x80x9d Bell Labs Technical Journal, January-March 1999, pp. 207-229). For example, WADMs have been constructed using optical multiplexer/demultiplexer pairs that first demultiplex a multi-channel WDM optical signal into individual WDM channels on individual optical paths, and then re-multiplex signals on the individual optical paths back into a single multi-channel WDM optical signal. Single channel WDM signals may be dropped from or added to a selected number of the individual optical paths before the signals are re-multiplexed.
Alternatively, in order to avoid demultiplexing and re-multiplexing each of the channels in the WDM signal, a variety of optical filter technologies have been employed in WADM systems to drop signals from or add signals to selected channels in the multi-channel WDM optical signal. Such filter technologies include, for example, fiber Bragg gratings (FBGs), thin film filters and arrayed waveguide gratings. Use of such filter technologies in WADMs is preferred when only a few of many channels in a WDM signal are either being dropped or added.
Optical filter characteristics are largely dictated by associated WDM signal characteristics. For example, synchronous optical network (SONET) OC192 channels operating at 10 gigabits per second require filters with an effective bandwidth of at least 48 gigahertz, while SONET OC48 channels operating at 2.5 gigabits per second require filters with an effective bandwidth of at least 10 gigahertz. In addition, OC192 channels require filters that are selective among channels spaced at 100 gigahertz intervals, while OC48 channels require filters that are selective among channels spaced at 50 gigahertz intervals. As a result, WADM filters usable at one WDM data transmission rate are generally unusable at alternate data rates.
For increased flexibility, some current WDM systems allow individual channels to be operated at alternate data rates. For example, an OC192 channel with 100 gigahertz spacing may alternatively be replaced by two OC48 channels with 50 gigahertz spacing. This increased flexibility helps to maximize utilization of capacity in WDM systems.
To date, such flexible systems have used dedicated WADM filters to filter signals at each data rate. This approach adds cost and reduces inherent flexibility in the selection of channels for a given WADM signal. Accordingly, there is a need to provide a more flexible and cost-effective means for filtering optical channels in a WDM signal with varying data rates.
Flexibility is increased and cost is reduced in an optical wavelength add/drop multiplexer (WADM) configured to add or drop two or more WDM channels that may each be operating at one of either a first data rate or a second data rate. The WADM comprises an optical circulator that is coupled at one port to two or more serially interconnected FBGs, and at another port to a thin film filter including two or more serially interconnected thin film filter elements (TFFEs). Each of the FBGs and TFFEs has an effective bandwidth to filter signals from one of the two or more WDM channels. Bandwidth and dispersion characteristics for the FBGs are selected to minimize anticipated filter performance penalties for operation at both the first and second data rates.
FBGs and TFFEs contribute insertion loss to the filtered signals. According to the principles of the present invention, FBGs and TFFEs are configured to approximately equalize the amount of insertion loss associated with each added or dropped channel. Specifically, FBGs and TFFEs are configured such that optical channels are assigned to FBGs in order of the FBGs"" increasing optical distance from the circulator, and assigned to TFFEs in order of the TFFEs"" decreasing optical distance from the circulator.
In a preferred embodiment of the invention supporting a first data rate of no more than 2.5 gigabits per second and a second data rate of 10 gigabits per second, the WADM includes four FBGs and four thin film filters. In order to employ conventional thin film filter elements having an effective bandwidth of 200 gigahertz, each pair of adjacent FBGs and each pair of adjacent thin film filters are selected to have characteristic wavelengths spaced at 200 gigahertz intervals. Bandwidth and dispersion characteristics of the FBGs are selected to enable operation at both the first and second data rates. Specifically, each FBG is selected to have an effective bandwidth (i.e., reflected by a power difference over the bandwidth of no more than 10 dB) of about 0.45 nanometers. Each FBG is further selected with dispersion values that deviate by no more than approximately 150 picoseconds per nanometer from a predetermined reference value at wavelengths no more than 0.1 nanometers above and below a characteristic wavelength, and with deviation increasing above 150 picoseconds per nanometer at a rate no greater than approximately 20,000 picoseconds per square nanometer at wavelengths beyond 0.1 nanometers from the characteristic wavelength.